Compelling Evidence That Wider Lanes Make City Streets More Dangerous

The rate of side impact crashes is lowest on urban streets with lanes about 10.5 feet wide — much narrower than the standard 12 feet. Graph: Dewan Karim

The “forgiving highway” approach to traffic engineering holds that wider is safer when it comes to street design. After decades of adherence to these standards, American cities are now criss-crossed by streets with 12-foot wide lanes. As Walkable City author Jeff Speck argued in CityLab last year, this is actually terrible for public safety and the pedestrian environment.

A new study reinforces the argument that cities need to reconsider lane widths and redesign streets accordingly. In a paper to be presented at the Canadian Institute of Traffic Engineers annual conference, author Dewan Masud Karim presents hard evidence that wider lanes increase risk on city streets.

Karim conducted a wide-ranging review of existing research as well as an examination of crash databases in two cities, taking into consideration 190 randomly selected intersections in Tokyo and 70 in Toronto.

Looking at the crash databases, Karim found that collision rates escalate as lane widths exceed about 10.5 feet.

Roads with the widest lanes — 12 feet or wider — were associated with greater crash rates and higher impact speeds. Karim also found that crash rates rise as lanes become narrower than about 10 feet, though this does not take impact speeds and crash severity into account. He concluded that there is a sweet spot for lane widths on city streets, between about 10 and 10.5 feet.

In Toronto, where traffic lanes are typically wider than in Tokyo, the average crash impact speed is also 34 percent higher, Karim found, suggesting that wider lanes not only result in more crashes but in more severe crashes.

The “inevitable statistical outcome” of the “wider-is-safer approach is loss of precious life, particularly by vulnerable citizens,” Karim concluded.

  • neroden

    Please think before you write. “Human factors” means human psychology, and we *know* some facts about human psychology: wide, straight roads cause people to drive faster. Traffic calming == narrow, twisty roads. Period.

  • neroden

    OK, that’s completely crazy in Maryland. The general rule is that traffic in the intersection has the right of way and traffic not-yet-in-the-intersection must yield to it. A roundabout is an intersection. Therefore traffic within the roundabout has the right of way and traffic not-yet-in-the-roundabout must yield to it.

  • neroden

    There are definitely exceptions. Consistent police enforcement can change typical speeds on expressways — for instance, there are quite a few with design speeds of 80 or 90, but when the national 55 mph speed limit was in effect and enforced, you saw very few people doing 80.

    This doesn’t seem to work at a scale as small as city streets, though. Posted speed limits seem to only really have an effect on behavior at very high speeds.

  • neroden

    All the Scandinavian countries index *all* fines to income, to make sure that fines have the correct punitive and deterrent effect. Here, by contrast, fines are very harsh for the poor and very mild for the rich. The Scandanavian system is the correct thing to do. We really ought to do it here.

  • neroden

    The fire department never, ever, ever needs larger trucks. Fire departments can afford to buy two smaller trucks rather than one gigantic truck — every time.

    Where I live, every time the fire department is called, at least four trucks show up anyway.

  • Phil Demosthenes

    Since you assume I don’t think before I write, I want to assure you that I most always think before I write. And I always try to reread and reconsider my statements before pushing the Post button. “Ergonomics (or human factors) is the scientific discipline concerned with the understanding of interactions among humans and other elements of a system, and the profession that applies theory, principles, data and methods to design in order to optimize human well-being and overall system performance”. I have never had a straight cause me to do something, But a straight road, smooth, with plenty of stopping sight distance allows me to do what I want to do – go faster and get there sooner. Depends on my assessment of risk. Why should I believe an open road with wide lanes has a hidden risk?

  • fdtutf

    Didn’t see the person until it was too late at night might often be true, but it means the driver was going too fast for conditions.

    THIS.

    Most drivers don’t understand the concept of adapting their speed to all the conditions that prevail, including darkness.

  • ahwr

    like maybe strong potential for children running into the street,

    Or if the block is 900 feet long and people might want to cross from the middle of one side of the street to the other.

    When the slopes start to wear off they end up being a major hazard for cyclists,

    Then leave a gap in the middle that bikes can use, but too narrow for cars.

  • Mike

    You answered your own question. A straight smooth road with plenty of stopping distance means that you will go faster regardless of the environment that you are driving in. So if you are in a driving environment with limited inputs (people, cars, etc) that’s fine. but when you’re on an urban street you should be driving slower and the road design should subconsciously cause you to drive slower.

  • Phil Demosthenes

    No, I did not say “regardless of the environment” and this is not true for most motorists. Most traffic calming techniques speak to the driver. The total context is part of the risk assessment. Its the design of the entire environment not just the design of the road. The environment is made up of many different elements. The engineering of the road is only one of the more important and influential elements. One must be careful when designing limitations to accomplish design, like a roundabout’s slow speeds, compared to using an engineered hazard to limit behavior like a speed bump. I live on a street with on-street parking and about 16 ft in the center for 2 direction travel. I travel slow until I get to a 11 ft lane collector. That’s fine with me. The context is correct and the speeds slow where the conflict potential is the greatest. But many accidents occur – loss of mirrors, scratches on sides of parked vehicles, taillights broken. It is not crash free where there is a tight slow street.

  • Mike

    I find that people seem to lose brain function on narrow streets when there is another car coming from the opposite direction-they either stop when there is room, hog the whole opening and rarely realize that they should slow down.

    Now with regards to driving quicker on 11′ collectors-that’s my point. Within the urban and urbanizing areas, lanes should be reduced in number and width. This will make the driver drive slower in general, even if they’re unaware of it. You shouldn’t “feel” that you can go higher speeds (30+mph) until you are on roads outside of urban areas. The conflict potential is greatest throughout the urban area, whether it be parked cars, pedestrians, or entrance/exits to shopping centers.
    By the way, I hate the whole concept of collector and arterial streets. They funnel traffic onto fewer roads causing more congestion not less. Then cities use that congestion as the reason to need more lanes, which destroys the urban fabric and perpetuates more driving because the areas adjacent to these roads become more hostile to peds and bicyclists.

  • Koen

    What happens if you simply narrow the streets, is that the bicyclists will bear the load during overtaking and at pinch points. You willhave to cater for cyclists as well as narrow the lanes, otherwise the effect will be zero to negative

  • adamold

    I don’t know how the signals work in NYC, but in Miami, pressing the little “cross street” button does exactly nothing to speed up the street crossing. (usually what it does is lengthen the time of crossing and add a white walking ped sign. If those buttons immediately triggered a light change (through yellow of course) peds would stop crossing against the light so frequently. Yes, it will probably mess up auto traffic flows. seems like a small price to pay.

  • adamold

    This would really be better with “Relative Index of *SERIOUS* Crash Rates.”

  • ahwr

    Pedestrians don’t always have to get a walk signal immediately, but shorter light cycles do improve compliance. I’ve seen some where the minimum time between legal pedestrian crossings is four minutes. That encourages risk taking. Elsewhere I’ve seen beg buttons that give cars a green time of thirty seconds, but after that time will turn yellow then red as soon as someone requests to cross, with very good compliance from pedestrians. This might be worth a look:

    http://nacto.org/usdg/intersection-design-elements/traffic-signals/signal-cycle-lengths/

    In NYC, for the most part lights run on fixed timers, with the minor street getting a green and pedestrians crossing the major street a walk phase even when nobody is there to cross and no cars are on the minor street. Interestingly, there are a lot of those beg buttons in NYC but many of them do absolutely nothing. They used to work, but when they switched many lights to a fixed cycle they didn’t want to spend the money to take the buttons out, just turned them off.

    http://www.nytimes.com/2004/02/27/nyregion/for-exercise-in-new-york-futility-push-button.html

  • Mike

    If it’s not a crash free street then it’s because of dumb drivers who haven’t figured out that you have to drive slower in tighter spots.

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